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LMZ14203TZE-ADJ 데이터시트(PDF) 11 Page - Texas Instruments |
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LMZ14203TZE-ADJ 데이터시트(HTML) 11 Page - Texas Instruments |
11 / 25 page LMZ14203 www.ti.com SNVS632N – DECEMBER 2009 – REVISED MARCH 2013 COT CONTROL CIRCUIT OVERVIEW Constant On Time control is based on a comparator and an on-time one shot, with the output voltage feedback compared with an internal 0.8V reference. If the feedback voltage is below the reference, the main MOSFET is turned on for a fixed on-time determined by a programming resistor RON. RON is connected to VIN such that on- time is reduced with increasing input supply voltage. Following this on-time, the main MOSFET remains off for a minimum of 260 ns. If the voltage on the feedback pin falls below the reference level again the on-time cycle is repeated. Regulation is achieved in this manner. Design Steps for the LMZ14203 Application The LMZ14203 is fully supported by Webench® and offers the following: Component selection, electrical and thermal simulations as well as the build-it board for a reduction in design time. The following list of steps can be used to manually design the LMZ14203 application. • Select minimum operating VIN with enable divider resistors • Program VO with divider resistor selection • Program turn-on time with soft-start capacitor selection • Select CO • Select CIN • Set operating frequency with RON • Determine module dissipation • Layout PCB for required thermal performance ENABLE DIVIDER, RENT AND RENB SELECTION The enable input provides a precise 1.18V band-gap rising threshold to allow direct logic drive or connection to a voltage divider from a higher enable voltage such as VIN. The enable input also incorporates 90 mV (typ) of hysteresis resulting in a falling threshold of 1.09V. The maximum recommended voltage into the EN pin is 6.5V. For applications where the midpoint of the enable divider exceeds 6.5V, a small zener can be added to limit this voltage. The function of this resistive divider is to allow the designer to choose an input voltage below which the circuit will be disabled. This implements the feature of programmable under voltage lockout. This is often used in battery powered systems to prevent deep discharge of the system battery. It is also useful in system designs for sequencing of output rails or to prevent early turn-on of the supply as the main input voltage rail rises at power- up. Applying the enable divider to the main input rail is often done in the case of higher input voltage systems such as 24V AC/DC systems where a lower boundary of operation should be established. In the case of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than the LMZ14203 output rail. The two resistors should be chosen based on the following ratio: RENT / RENB = (VIN UVLO/ 1.18V) – 1 (1) The LMZ14203 demonstration and evaluation boards use 11.8k Ω for RENB and 68.1kΩ for RENT resulting in a rising UVLO of 8V. This divider presents 6.25V to the EN input when the divider input is raised to 42V. The EN pin is internally pulled up to VIN and can be left floating for always-on operation. OUTPUT VOLTAGE SELECTION Output voltage is determined by a divider of two resistors connected between VO and ground. The midpoint of the divider is connected to the FB input. The voltage at FB is compared to a 0.8V internal reference. In normal operation an on-time cycle is initiated when the voltage on the FB pin falls below 0.8V. The main MOSFET on- time cycle causes the output voltage to rise and the voltage at the FB to exceed 0.8V. As long as the voltage at FB is above 0.8V, on-time cycles will not occur. The regulated output voltage determined by the external divider resistors RFBT and RFBB is: VO = 0.8V * (1 + RFBT / RFBB) (2) Rearranging terms; the ratio of the feedback resistors for a desired output voltage is: RFBT / RFBB = (VO / 0.8V) - 1 (3) These resistors should be chosen from values in the range of 1.0 kohm to 10.0 kohm. Copyright © 2009–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LMZ14203 |
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